Don't show the starting second of a PITR range which can't be used for PITR restore

For a full snapshot it has a starting time (which is shown as its name) and the end time which is also the consistent time.

For an example, using the current version 1.5.0 pbm list output, see below.

Backup snapshots:
  2021-03-14T12:11:42Z [complete: 2021-03-14T12:12:04]

PITR <off>:
  2021-03-14T12:12:04 - 2021-03-14T12:25:39

In this deployment PITR is enabled and the PITR span starting from that first and only snapshot can be seen with the starting time 2021-03-14T12:12:04, exactly the same as the ending and consistent time of the full snapshot.

If that time is used as the --time argument in "pbm restore --time xxxx" it will currently error. PITR can be specified to be one second after that.

Doing "pbm restore 2021-03-14T12:11:42Z" in this case would restore to end consistent time == the first second in the shown PITR span.

The current behaviour is frustrating for users. Let's address.